Covalent heterogenization of a discrete Mn(II) bis-phen complex by a metal-template/metal-exchange method: an epoxidation catalyst with enhanced reactivity

Abstract

Considerable attention has been devoted to the immobilization of discrete epoxidation catalysts onto solid supports due to the possible benefits of site isolation such as increased catalyst stability, catalyst recycling, and product separation. A synthetic metal-template/metal-exchange method to imprint a covalently attached bis-1,10-phenanthroline coordination environment onto high-surface area, mesoporous SBA-15 silica is reported herein along with the epoxidation reactivity once reloaded with manganese. Comparisons of this imprinted material with material synthesized by random grafting of the ligand show that the template method creates more reproducible, solution-like bis-1,10-phenanthroline coordination at a variety of ligand loadings. Olefin epoxidation with peracetic acid shows the imprinted manganese catalysts have improved product selectivity for epoxides, greater substrate scope, more efficient use of oxidant, and higher reactivity than their homogeneous or grafted analogues independent of ligand loading. The randomly grafted manganese catalysts, however, show reactivity that varies with ligand loading while the homogeneous analogue degrades trisubstituted olefins and produces trans-epoxide products from cis-olefins. Efficient recycling behavior of the templated catalysts is also possible.

Figure 1

X-band EPR signals in 2-methyltetrahydrofuran/propionitrile (2/1) at −196 °C of (a) Cu^IIT (0.1 mmol g⁻¹ 1^C), (b) Cu^IIT (0.1 mmol g⁻¹ 1^C) with 2 equiv of (n-Bu₄N)Cl per copper, (c) [Cu^II(2)₂](OTf)₂ (1 mM), (d) Cu^IIG (0.08 mmol g⁻¹ 1^C), and (e) Cu^IIG (0.08 mmol g⁻¹ 1^C) with 2 equiv of (n-Bu₄N)Cl per copper.

Figure 2

Conversion of 1-octene to 1,2-epoxyoctane by Mn^IIT (●) (0.11 mmol g⁻¹ 1^C) and Mn^IIG (■) (0.10 mmol g⁻¹ 1^C) in MeCN at 2 °C with 1.5 equiv of PAA.

Scheme 1

Schematic Representation of the Covalent Attachment of 1^H to the SBA-15 Silica via Random Grafting and Metal-Templating Methods To Form Mn^IIG and Mn^IIT, Respectively^{a a} (a) Random grafting of 1^H, (b) metalation with an excess of [Mn^II(CF₃SO₃)₂] per 1^C followed by washing with CH₃OH, (c) formation of [CuI(1H)₂]⁺ followed by covalent attachment, (d) demetalation with Et₂NCS₂Na.

Scheme 2